Nuclear charged self-sustaining laser

ABSTRACT

A nuclear charged solid state laser for which no external power supply is required. A large class of single crystals are disclosed which contain a stable isotope that is potentially radioactive. The crystal is doped with a laser impurity ion and is irradiated with nuclear energy such that the stable isotope becomes radioactive to provide a pumping source for the laser impurity ions therein. Optical resonator means can be provided by coating both ends of the crystal with highly reflective surfaces. The radioactive isotope provides a continuous internal power supply over the lifetime of its decay, thus providing a potentially compact, portable and inexpensive solid state laser device.

United States Patent Morrison et al. 1 May 30, 1972 54] NUCLEAR CHARGEDSELF- OTHER PUBLICATIONS SUSTAINING LASER Derr et al., Radio Isotopesfor Aerospace, ed. by Dempery et [72] Inventors: Clyde A. Morrison,Wheaton; Donald E. Plenum Press New York 1966 PP 312 Wortman, Rockville;Ruben T. Farrar, Wheaten, a of MCL Primary Examiner-Ronald L. WlbertAssistant Examiner-Edward S. Bauer Assignefi The United states ofAmerica 85 Att0mey-Harry M. Saragovitz, Edward J. Kelly, Herbert Ber]represented by the Secretary of the Army d J D. Edgerton [22] Filed:Aug. 17, 1970 ABSTRACT [2]] Appl' 64426 A nuclear charged solid statelaser for which no external power supply is required. A large class ofsingle crystals are 52 U.S.Cl ..331 94.s disclosed which contain astable isotope that is potentially 511 Im. Cl ..H01s 3/09 radioactiveThe crystal is doped with a laser p y ion and [58] Field of Search ..331/94.5 is irradiated with nuclear energy Such that the stable isotope Ybecomes radioactive to provide a pumping source for the laser [56]References Cited impurity ions therein. Optical resonator means can beprovided by coating both ends of the crystal with highly reflectiveUNITED STATES PATENTS surfaces. The radioactive isotope provides acontinuous internal power supply over the lifetime of its decay, thusproviding 3,391,281 7/l968 Eerkens ..33l/94.5 X a potentially compact,portable and inexpensive So'id state 3,559,095 1/1971 Ntelson ..33l/94.5laser device 3,309,622 3/1967 Weiner et al. ..33 l/94.5 3,470,490 9/1969Held et al. ..33 l/94.5 13 Claims, 2 Drawing Figures NUCLEAR CHARGEDSELF-SUSTAINING LASER RIGHTS OF GOVERNMENT The invention describedherein may be manufactured, used,

,and licensed by or for the United States Government for governmentalpurposes without the payment to us of any royalty thereon.

BACKGROUND OF THE INVENTION This information relates to lasers, and moreparticularly, to solid state lasers energized by nuclear radiation.

It is old in the art to provide a coherent light source or generatorsuch asan artificial ruby crystal with an excitation source of pumpinglight such as a flash lamp. The size of the pumping energy source forsuch generators is limited unless it is contemplated to provide apermanent installation rather than a portable installation for acoherent light source. It would obviously be desirable if one couldavail oneself of the large energy and power inherent in devices known asnuclear reactors rather than conventional energy sources derived fromthe conversion of water or coal power to electricity, as pointed I outand disclosed in U.S. Pat. No. 3,470,490 to Held et al. The

foregoing patent discloses the use of nuclear energy for effectingcoherent light generation in .which the ionizing radiation must firstinteract with a scintillating phosphor, and then have the output of saidphosphor pump the laser material. The disadvantages and limitationsinherent in this type of device include the requirement for an externalnuclear reactor, the requirement for multiple energy transduction, andthe requirement for overlapping radiation wavelength bands of thephosphor transducer and the laser material. Additionally, the size andcost of a suitable nuclear reactor to power the foregoing deviceseverely limits its portability and use.

, It is therefore a primary object of the present invention to provide asingle crystal laser device, which is completely selfcontained in thatno external power supply is required for the operation of the laser.

Another object of the present invention is to provide a solid statelaser device which contains a radioactive isotope that is directlyutilized as a pumping source for laser impurity ions therein.

Q A further object of the present invention is to provide a nuclearcharged solid state laser which can be made extremely small in size toprovide a compact, portable, self-contained device.

SUMMARY OF THE INVENTION Briefly, in accordance with the invention, asolid state laser device is provided which contains its own radioactivesource of energy. A good optical quality material is doped with laserimpurity ions and contains a stable isotope that is irradiated withnuclear energy such that the isotope becomes radioactive and provides apumping source for the laser impurity ions. The material, which can beof a single crystal structure, will provide a laser output in accordancewith the emission wavelength of the laser impurity ions that isproportional to the half-life of the radioactive isotope therein. Thecompactness in size with which the device can be constructed lendsitself to a degree of portability that has been heretofore unachieved.

-.BRIEF DESCRIPTION OF THE DRAWINGS The specific nature of the inventionas well as other objects, aspects, uses, and advantages thereof willclearly appear from the following description and from the accompanyingdrawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the development of thedevice of the present invention one begins with a good optical qualitymaterial that is doped with laser impurity ions and contains a stableisotope that is potentially radioactive. An example of such a material,which is grown as a single crystal scheelite structure, is the compoundlithium thulium flouride, LiTmF,. This crystal can be grown doped with asmall amount of neodymium, which has been observed to lase at 1.06microns. Neodymium is but one of a number of rare earth ions which haveexhibited laser action at various wavelengths and which can be grown ina crystal of the type envisioned in the device of the present invention.The stable isotope selected for the foregoing example is thulium, whichoccurs naturally as the single isotope thulium 169 and has a thermalneutron capture cross-section of approximately barns. The cut andoptically polished crystal is placed in a nuclear reactor, having a fluxof approximately l0 thermal neutrons/cm sec., for a period ofapproximately a month. The thulium 169, by virtue of its large capturecrosssection, tends to capture a neutron and becomes thulium which isradioactive. The thulium 170 upon decaying via either of two branches bythe emission of an electron becomes ytterbium 170 which is stable. Thistransition is accompanied by the emission of beta and gamma rays whichare used, in turn, to pump the lasing material within the crystal.Emission of 84 kev gamma rays follows the thulium decay to an excitedstate of the ytterbium 170. Approximately 23 percent of the betaemission goes to this excited state with a half-life of 1.59 X 10" sec.and about 77 percent of the beta decay goes to the ground state.

The crystal is then removed from the reactor and placed in a suitableoptical resonator as shown in FIG. I. The fully charged crystal 10 isheld in place by crystal supports 20 and 22. A partially reflectingmirror 12 and a totally reflecting mirror 14 are placed at opposite endsof crystal 10 and are held in place by mirror support members '16 and18. The half-life of the radioactive thulium 170 is approximately 129days which is ample time to transport the crystal from a reactor to aparticular location where it is needed. The resultant laser structure isseen to be compact and portable, in that it requires no external powersupply once it has been charged in the reactor. Alternatively, as seenin FIG. 2, the fully charged crystal 10 can be coated on its ends with atotally reflecting surface 26 and a partially reflecting surface 24 toprovide an even more compact and portable unit. The finished device willlase continuously for a period of time proportional to the half-life ofthe radioactive isotope therein. It is understood that various externaldevices can be used to modulate the output of the laser in any desiredmanner. Some of thefactors that influence the selection of a stableisotope to be utilized in the device of the present invention are thatthe isotope should have:

1. a large capture cross-section for thermal neutrons,

2. a long half-life decay, and

3. a nonlethal decay energy.

An obvious advantage of the device of the present invention is that itsweight and size can be less than a conventional twocell flashlight. Infact, the laser as shown in FIG. 2 can be made less than A inch indiameter and 1 inch long. Another advantage in utilizing the foregoingexample is that the thuliurn 169 ion occurs 100 percent naturally and isthus quite inexpensive. It is also understood that in constructing thedevice of the present invention, one may work initially with theradioactive thulium 170 and grow a crystal that is the finished product,thus avoiding the need for placing the crystal in a nuclear reactor.While neodymium was specified as the laser impurity ion for theforegoing example, any rare earth ion can be utilized according to thedesired laser wavelength output.

There exists many single crystal structures which will suffice as a goodoptical quality material for the device of the present invention. Amongthese are the vanadates, arsenates, phosphates, triflourides,tungstates, molibdates, .and aluminates. Other crystalline structuresthat would prove useful are the scheelites, zircons, garnets, andmonoclinically distorted scheelites. While the single crystal structureappears to be most advantageous, other noncrystalline substances may beutilized as the host material, such as glass. It is seen that we haveprovided a compact and portable self-sustaining laser in which noexternal power supply is required for its operation. The power forpumping the laser is supplied by an internal decay of a radioactiveisotope. The radioactive isotope can be the same chemical element as thelaser impurity ion.

We wish it to be understood that we do not desire to be limited to theexact details of construction shown and described, for obviousmodifications will occur to a person skilled in the art.

We claim as our invention:

l. A nuclear charged self-sustaining laser, comprising:

a. a single crystal material capable of transmitting coherent light;

b. laser impurity ions located within said crystal, said ions comprisinga rare earth;

c. means for pumping said laser impurity ions, said pumping meanscomprising a constituent of said crystal, said constituent comprisingthulium 169 which is irradiated with thermal neutrons, becoming thulium170 which pumps said laser impurity ions by virtue of its radioactivedecay to ytterbium 170; and

d. an optical resonator for initiating and sustaining laser emissionwithin said crystal.

2. The invention according to claim 1 wherein said single crystal isselected from the group consisting of vanadates, arsenates, phosphates,triflourides,tungstates, molibdates, and aluminates.

3. The invention according to claim 2 wherein said rare earth comprisesneodymium.

4. The invention according to claim 1 wherein said single crystal isselected from the group consisting of scheelites, zircons, garnets andmonoclinically distorted scheelites.

5. The invention according to claim 4 wherein said scheelite crystalcomprises lithium thulium flouride.

6. The invention according to claim 5 wherein said rare earth comprisesneodymium.

7. The invention according to claim 1 wherein said optical resonatorcomprises a highly reflective coating that is applied on both ends ofsaid crystal.

8. The invention according to claim 1 wherein said optical resonatorcomprises a pair of highly reflective mirrors located at either end ofsaid crystal.

9. A nuclear charged self-sustaining laser, comprising:

a. a single crystal material capable of transmitting coherent light;

b. laser impurity ions located within said crystal, said ions comprisinga rare earth;

c. means for pumping said laser impurity ions, said pumping meanscomprising a constituent of said crystal, said constituent comprisingthulium which pumps said laser impurity ions by virtue of itsradioactive decay to ytterbium 170; and

d. an optical resonator for initiating and sustaining laser emissionwithin said crystal.

10. A method of fabricating a self-sustaining laser, comprising thesteps of:

a. providing a single crystal material capable of transmitting coherentlight that is doped with rare earth laser impurity ions and has as aconstituent thereof a stable isotope comprising thulium 169; i

b. cutting and polishing the ends of said material to create a rod ofgood laser quality;

c, irradiating said material with thermal neutrons such that saidthulium 169 becomes thulium 170 which provides a pumping source for saidlaser impurity ions by virtue of its radioactive decay to ytterbium 170;and

d. providing said irradiated material with optical resonator means toinitiate and st stain l ser oscillation therein.

2. The invention according to claim 1 wherein said single crystal isselected from the group consisting of vanadates, arsenates, phosphates,triflourides, tungstates, molibdates, and aluminates.
 3. The inventionaccording to claim 2 wherein said rare earth comprises neodymium.
 4. Theinvention according to claim 1 wherein said single crystal is selectedfrom the group consisting of scheelites, zircons, garnets andmonoclinically distorted scheelites.
 5. The invention according to claim4 wherein said scheelite crystal comprises lithium thulium flouride. 6.The invention according to claim 5 wherein said rare earth comprisesneodymium.
 7. The invention according to claim 1 wherein said opticalresonator comprises a highly reflective coating that is applied on bothends of said crystal.
 8. The invention according to claim 1 wherein saidoptical resonator comprises a pair of highly reflective mirrors locatedat either end of said crystal.
 9. A nuclear charged self-sustaininglaser, comprising: a. a single crystal material capable of transmittingcoherent light; b. laser impurity ions located within said crystal, saidions comprising a rare earth; c. means for pumping said laser impurityions, said pumping means comprising a constituent of said crystal, saidconstituent comprising thulium 170 which pumps said laser impurity ionsby virtue of its radioactive decay to ytterbium 170; and d. an opticalresonator for initiating and sustaining laser emission within saidcrystal.
 10. A method of fabricating a self-sustaining laser, comprisingthe steps of: a. providing a single crystal material capable oftransmitting coherent light that is doped with rare earth laser impurityions and has as a constituent thereof a stable isotope comprisingthulium 169; b. cutting and polishing the ends of said material tocreate a rod of good laser quality; c. irradiating said material withthermal neutrons such that said thulium 169 becomes thulium 170 whichprovides a pumping source for said laser impurity ions by virtue of itsradioactive decay to ytterbium 170; and d. providing said irradiatedmaterial with optical resonator means to initiate and sustain laseroscillation therein.